Have you ever used a hand warmer at a winter game or an instant cold pack after a sports injury? In just a few seconds, a small packet suddenly becomes hot or cold without being plugged in or put in a freezer. That is not magic—it is chemistry and engineering working together. In this lesson, you will learn how to think like a scientist and an engineer to design a device that uses chemical reactions to release or absorb thermal energy.
To design a good heat-releasing or heat-absorbing device, you first need to understand what a chemical reaction is.
A chemical reaction happens when substances (called reactants) interact and form new substances (called products). The tiny particles that make up substances are atoms. Atoms can join together to form molecules.
During a chemical reaction:
This means the number of each type of atom is conserved (stays the same) before and after the reaction. For example, if you start with a molecule that has 2 hydrogen atoms and 1 oxygen atom, you will still have 2 hydrogen atoms and 1 oxygen atom in total after the reaction—they might just be in different molecules.
Even though atoms are conserved, the energy of the system can be stored or released in different ways. The bonds between atoms store chemical energy. When bonds break and new ones form, energy can move in or out of the reaction.
Two important types of reactions related to energy are:
Thermal energy is related to how fast particles are moving. When particles move faster on average, the temperature is higher. When they move more slowly, the temperature is lower.
When a chemical reaction happens, energy can move between the reacting substances (the system) and everything else around them (the surroundings). Exothermic and endothermic reactions differ by the direction of this energy flow, as illustrated in [Figure 1].
Exothermic reactions:
Examples of exothermic processes include:
Endothermic reactions:
Examples of endothermic processes include:
In both exothermic and endothermic reactions, the atoms are rearranged, but conserved. What changes is how energy is stored in the bonds and how much thermal energy the surroundings gain or lose.
To create a device that uses a chemical reaction to heat or cool something, engineers follow a process. You can use the same steps.
The engineering design process often includes:
Your goal is to design a device that either:
To stay focused, you will judge your design using simple, clear criteria: how the temperature changes, how much substance you use, and how long (time) the device can keep heating or cooling.
A common real-world example of an exothermic heat device is a chemical hand warmer. These are used by hikers, skiers, and outdoor workers. Many commercial hand warmers use iron powder that reacts with oxygen in the air and slowly releases thermal energy.
You can design a simplified version of a heat-releasing device for learning, as shown in [Figure 2]. A typical design might include:
When the reaction starts, atoms in the reactants rearrange to form new products. The reaction releases thermal energy, which moves into the surrounding materials in the bag and then into the air or into your hand holding it. To you, it feels warm.
When designing this kind of device, you can think about:
Even though you may not write chemical equations at this level, remember that in any exothermic reaction inside your device, the number of atoms of each element is the same before and after. What changes is how they are grouped into molecules and how much energy is stored in their bonds.
Now think about an instant cold pack you might use after a sprained ankle. Inside, there is usually a solid and a liquid separated. When you squeeze the pack, they mix, and the pack quickly becomes cold. The reaction inside is endothermic, meaning it absorbs thermal energy from its surroundings—your skin and the air. To you, the pack feels cold.
For a learning-level cooling device, you might consider reactions or processes that are safe and easy to handle, such as:
A simple design concept could include:
When you mix the solid and the water, bonds in the solid and water molecules change. The process needs extra energy, so it pulls thermal energy from the surroundings. That makes the surroundings cooler.
Again, the atoms do not disappear. For example, if your solid contains nitrogen, hydrogen, and oxygen atoms, those same atoms are present afterward, now arranged in particles dissolved in water.
Design choices you can explore include:
To see whether your device design works well, you need to test it in a careful, scientific way.
You collect data and look at patterns over time. This section focuses on the key criteria for judging your design: amount of substance, time, and temperature, as illustrated in [Figure 3].
1. Measuring Temperature
You will use a thermometer to measure how hot or cold something is. Temperature is usually measured in degrees Celsius, written as \(^{\circ}\textrm{C}\).
To test a heat device:
2. Time and Temperature Change
During testing, you might make a data table like the one shown in [Figure 3], where each row shows the time and the temperature at that time. For example, for a heat-releasing device, you might see the temperature rise from \(20^{\circ}\textrm{C}\) to \(32^{\circ}\textrm{C}\) over 10 minutes. For a cooling device, you might see it drop from \(22^{\circ}\textrm{C}\) to \(15^{\circ}\textrm{C}\).
You can describe the temperature change using \(\Delta T\), which is read as “delta T” or “change in temperature.” It is calculated by:
\[\Delta T = T_{\textrm{final}} - T_{\textrm{initial}}\]For example, if a hand warmer starts at \(20^{\circ}\textrm{C}\) and reaches \(32^{\circ}\textrm{C}\), then:
\[\Delta T = 32^{\circ}\textrm{C} - 20^{\circ}\textrm{C} = 12^{\circ}\textrm{C}\]This tells you the temperature increased by \(12^{\circ}\textrm{C}\).
If a cold pack starts at \(22^{\circ}\textrm{C}\) and cools to \(15^{\circ}\textrm{C}\), then:
\[\Delta T = 15^{\circ}\textrm{C} - 22^{\circ}\textrm{C} = -7^{\circ}\textrm{C}\]The negative sign shows the temperature went down by \(7^{\circ}\textrm{C}\).
3. Amount of Substance
You can also test how the amount of reactants affects the temperature change. For example, you might compare:
If you keep everything else the same (same amount of water, same container, same starting temperature, same time), you can see how the amount of substance influences \(\Delta T\).
4. Graphing (Conceptual)
If you plotted your data on a graph, with time on the horizontal axis and temperature on the vertical axis, you would see a line going up for exothermic devices and a line going down for endothermic devices. A steeper line means the temperature is changing faster.
Once you have tested your device and recorded your data, you use that information to make it better. This is where engineering and science come together again. You might look back at your graph and data like in [Figure 3] to help decide what to change.
Here are some ways you might modify your device while still focusing on amount, time, and temperature:
Every time you change just one variable, you test again and compare the temperature vs time data with your previous tests. You are using evidence to make design decisions, just like real engineers.
While doing this, remember that inside your device, the chemistry is still following the same rules:
Many technologies around you use the same science and engineering ideas as your classroom devices.
1. Commercial Hand Warmers
Most air-activated hand warmers you can buy at a store contain:
When you open the package, oxygen from the air reaches the iron powder. The iron atoms react with oxygen atoms to form iron oxide (rust). The atoms are rearranged but conserved, and the reaction is exothermic. The device releases thermal energy for several hours.
2. Instant Cold Packs
Many instant cold packs contain water and a solid chemical in separate regions. When you squeeze the pack, the barrier breaks and the solid dissolves in water. The dissolving process is endothermic and absorbs thermal energy from your skin, cooling it. The atoms of the solid and water are still all there; they are just arranged differently in the dissolved state.
3. Self-Heating and Self-Cooling Food Packages
Some military food rations and camping meals use built-in chemical heaters. When you add water to a packet and place it under the meal, an exothermic reaction warms the food. Similarly, some drink cans have built-in cooling devices based on endothermic processes. These devices are designed using the same principles you are learning: chemical reactions, energy transfer, and careful measurement of temperature over time.
• Chemical reactions rearrange atoms. Reactants turn into products, but the number of atoms of each element stays the same. Atoms are conserved.
• Energy changes in reactions. In exothermic reactions, thermal energy is released and the surroundings warm up. In endothermic reactions, thermal energy is absorbed and the surroundings cool down.
• Devices can use these reactions. Hand warmers, cold packs, and other devices are designed to control exothermic or endothermic reactions to provide useful heating or cooling.
• The engineering design process helps improve devices. You define a problem, imagine solutions, plan, build, test, and improve.
• Testing focuses on amount, time, and temperature. You judge how well your device works by changing the amount of substance, measuring temperature changes, and watching how those changes happen over time.
• Data guides better designs. By recording temperature vs time and comparing \(\Delta T\) for different tests, you can decide how to modify your device to make it heat or cool more effectively.
By combining an understanding of chemical reactions and energy with careful measurement and design, you can create and improve real devices that release or absorb thermal energy, just like the ones used in sports, medicine, and outdoor activities.
